Actuator system

Abstract

An actuator having a compressive element 2 (e.g. bladder, bellow, hollow tube) defining an internal cavity 4 along a compression axis 14, means for applying compressive force to the compressive actuator 2, and a pump pressurising the internal cavity to increase the length of the actuator. The compressive element may be taller along the axis than wide perpendicular to the axis, providing a maximum actuation distance along the axis longer than the width of the compressive element. The compressive element may include two shells joined by their respective annular ring 84 (fig 12). The compressive element may be polymer. The actuator may exhibit a first stiffness below a first stress threshold, a second lesser stiffness between the first and second stress threshold, and a third lesser stiffness above the second stress threshold. The actuator may include valves and rigid endplates (fig 3-7). Multiple actuators may be arranged in series uniaxially.

Description

BACKGROUND OF THE INVENTION This invention relates to an actuator system and a method of actuating movement of an external component using an actuator system. It is known to provide unidirectional pushing or pulling force using a rigid hydraulic cylinder in order to actuate movement of an external object or component. Hydraulic cylinders are driven by pressurized hydraulic fluid, which is incompressible. Such hydraulic cylinders require a large number of parts and in particular require a number of seals in order to function. Moreover, they are driven by hydraulic fluid which may pose a contamination risk to the environment if the seals fail. The requirement for a large number of parts, and for separate hydraulic fluid to be provided, also makes hydraulic rams relatively costly. It is separately known to lift objects from below using inflatable lifting bags, which are inserted under the object to be lifted in a flat, uninflated state and are then inflated using pressurized air. These bags must withstand very large internal pressures in order to lift large objects, and they therefore often have a very short fatigue life. Furthermore, these bags have a very short stroke (i.e. lifting distance) relative to their overall size. In many circumstances, such actuation systems are inconvenient or may present certain disadvantages. The present invention seeks to provide an actuator system which addresses at least some of these shortcomings. SUMMARY OF THE INVENTION From a first aspect, the invention provides an actuator system, comprising: a compressive element, extending along a primary compression axis and configured for compression along the primary compression axis, the compressive element defining an internal cavity extending along the primary compression axis; a compression means, arranged to apply a compressive force to the compressive element, along the primary compression axis; and a pump, arranged to pump fluid into the internal cavity to increase the length of the compressive element along the primary compression axis. From a second aspect, the invention provides a method of actuating movement of an external component using an actuator system, the method comprising: compressing a compressive element by applying a compressive force to the compressive element along a primary compression axis along which the compressive element is configured for compression; and pumping fluid into an internal cavity of the compressive element to increase the length of the compressive element along the primary compression axis. The compressive element is acted on by a compressive force. Thus, the compressive element is compressed from its maximum length (i.e. its uncompressed length) to a length which is less than its maximum (uncompressed) length. Thus, any internal pressure created within the cavity by the pump pumping fluid into the internal cavity acts to increase the length of the compressive element along the primary axis, and therefore acts against the compressive force applied by the compression means. Thus, it will be seen that, in accordance with the invention, by pumping fluid into an internal cavity of a compressive element, the compressive element is made to increase in length along the primary compression axis, acting against the compressive force, and this change in length is usable to provide actuation either directly or via a connection means (e.g. a rope attached to the actuator system), to an external component, so as to move a weight. Since the compressive element already applies an outwards reaction force (due to its compression) a relatively small internal pressure is sufficient, when combined with this reaction force, to cause expansion of the internal cavity against the compressive force. This allows actuation to be achieved with a relatively low internal pressure. Providing this actuation with a compressive element, arranged to be (independently) compressible along the primary compression axis is advantageous because it ensures that the decompression provided by pumping in fluid is mostly (or entirely) converted into a change in length along the compression axis, thus providing actuation efficiently. Since the compressive element compresses or expands relatively little along directions other than the primary compression axis, the changes in cavity pressure caused by the pump are not “wasted” causing expansion or compression along other directions, which would lessen the actuation distance. The pre-compression by the compressive force allows the compressive element to be expanded (by applying an additional force using supplied internal pressure) to achieve actuation, rather than needing to always contract the compressive element by drawing out fluid, which may be more challenging to achieve (since there is clearly a lower limit to how far pressure can be reduced than increased). Since the compressive element is configured for compression, it provides a responsive force under compression, independently of the fluid contained within it. This is advantageous since it allows the actuation of the actuator system to be predictably controlled, based on selecting and / or knowing the response of the compressive element under compression. When there is no internal pressure within the internal cavity (e.g. when the fluid pressure within the compressive element is approximately equal to a fluid pressure outside of the compressive element) the compressive force is acted against only by the reaction force produced by the compressive element under the compression distance which it is compressed to by the compressive force. The compressive element is compressed to the compression distance at which the response force of the compressive element balances the compressive force. Thus, by selection of the compressive force, the initial compression distance of the compressive element may be selected. The achievable actuation distance is set by the possible stroke distance of the actuator, which will be the difference between its compressed length (due only to the compressive force) and its uncompressed length. The compressed length can be selected by choice of the magnitude of the compressive force, and therefore the stroke length may be selected as required. Furthermore, a compressive element, which is designed for compression along the primary compression axis, will have a very good fatigue life under repeated compression and expansion for purposes of actuation (e.g. compared to a non-compressive element, e.g. an expanding chamber). The compressive element is configured for compression along the primary compression axis. By being “configured for compression” it will be understood that the compressive element is arranged such that when a compressive stress is applied to the compressive element substantially along the primary compression axis, the compressive element is compressed along that axis (i.e. its length along that axis reduces). Thus, the compressive element changes length along the primary compression axis in response to a compressive force applied along that axis, and in doing so produces a response force, responsive to the applied compression. Further details of the response curve (i.e. force produced relative to compression distance) provided by the compressive element are described below. It will be appreciated that this response curve is the response of the compressive element itself, i.e. independently of any force provided due to fluid contained within the internal cavity. Thus, it is the compressive element itself which is configured for compression, to vary in length along the primary compression axis in response to a compressive force, independently of fluid contained within the internal cavity. The compressive element is configured for compression specifically along the primary compression axis. By this it will be understood that the compressive element is arranged (e.g. shaped) to be preferentially compressible along a particular axis, referred to as the primary compression axis (e.g. like a spring, which is primary compressible along a central axis and designed for compression along that axis). Thus, the compressive element is much more compressible (i.e. changes length in response to a compressive force) along that axis more significantly than along any other (e.g. perpendicular) axes. The compressive element is therefore less stiff along the primary compression axis than in directions perpendicular to the primary compression axis. This is further advantageous since the preferential compression axis makes the actuator system less likely to exhibit any diversion from the actuation direction, e.g. lean. The internal cavity extends along the primary compression axis. By this it will be understood that the internal cavity of the compressive element has a non-zero length along the primary compression axis when the compressive element is in a neutral position, i.e. when no external compressive forces are acting on the compressive element and when no internal forces due to fluid within the internal cavity are acting on the compressive element. Provided that the internal cavity also extends perpendicular to the primary compression axis when the compressive element is in a neutral position, the internal cavity may have a non-zero volume in the neutral configuration. Thus, the internal cavity is not only given shape and volume by a fluid contained within it, but rather has this independently. The pump is arranged to pump fluid into the internal cavity to alter (i.e. increase) the length of the compressive element. By this it will be understood that the pump is arranged to pump fluid into the internal cavity so as to decompress the compressive element along the primary compression axis (i.e. acting against the compressive force). The pump may only need to pump fluid in one direction (e.g. only into the internal cavity) since depending on the pressure differential between the internal cavity and the external environment in which the actuator is situated, it may be possible for fluid to flow in the direction out of the internal cavity without the need for a pump (e.g. simply by opening a valve fluid may flow out, where the pressure differential favours that direction of flow). Of course, in some embodiments, the pump is arranged to both pump fluid into the internal cavity to increase the length of the compressive element along the primary compression axis, and to pump fluid out of the internal cavity to decrease the length of the compressive element along the primary compression axis (i.e. it can pump in both directions). Thus, in some embodiments, the pump is also arranged to pump fluid out of the internal cavity to decrease the length of the compressive element along the primary compression axis. This advantageously improves the versatility of operation of the actuator. In some embodiments, the compressive element is pre-compressed before use. By this it is meant that in setting up the actuator system, the compressive force is applied to compress the compressive element to the compressed length, before the actuator system is used for actuation. In some embodiments, the compressive element extends at least as far along the primary compression axis as it extends perpendicular to the primary compression axis (i.e., in a neutral, uncompressed, unpressurised condition). The distance that the compressive element extends along the primary compression axis (in the uncompressed configuration) may be referred to as its length. The distance that the compressive element extends along a direction (optionally all directions) perpendicular to the primary compression axis may be referred to as its width, or diameter. Thus, its length may be larger than its diameter, optionally at least twice as large. In some embodiments, the compressive element is elongate along the primary compression axis (i.e. it extends further, optionally substantially further, along the compression axis than along directions perpendicular to the compression axis). This extension along the primary compression axis is advantageous since it enables larger increases or decreases in length of the compressive element along this direction, improving actuation range. In some embodiments the actuator system is arranged to provide a maximum actuation distance along the primary compression axis which is larger than the width (or diameter) of the compressive element perpendicular to the primary compression axis. The maximum actuation distance may also be referred to as the stroke length. It will be appreciated that the stroke length is equal to the difference between the length of the compressive element when uncompressed, and the length of the compressive element when compressed by the compressive force, absent any forces due to internal pressure within the internal cavity (i.e. where only the response force balances the compressive force). The length of the compressive element may be over 50 cm, optionally over 1 m, further optionally over 1.5 m, further optionally over 2 m. The diameter of the compressive element may be over 50 cm, optionally over (or approximately) 1 m. It will be understood that compressive elements of this scale are able to withstand large forces, such that the actuation system is able to provide very large actuation forces, e.g. of at least 150 - 200 tonnes. These parts may be moulded in individual pieces (e.g. shells as discussed below) and welded together to seal the compressive element together. The compressive element may be a spring. The compressive element may be a hollow tube (i.e. the hollow part defining the internal cavity). The tube may have a shaped or curved profile, along the primary compression axis. In some embodiments, the compressive element comprises at least one bellow (e.g. an outwardly protruding portion, protruding along a direction substantially perpendicular to the primary compressive axis), i.e. defined by the shaped or curved profile of the hollow tube. The compressive element may comprise a plurality (e.g. two or three) bellows. In some embodiments, the compressive element comprises at least two shells, each shell may comprise a first annular portion, a second annular portion and a central section, wherein the first annular portion and the second annular portion each lie in a plane that is substantially perpendicular to the primary compression axis, wherein the first annular portion has a maximum dimension in a direction substantially perpendicular to the compression axis that is greater than a maximum dimension of the second annular portion in a direction substantially perpendicular to the primary compression axis, and wherein the central section connects and extends between the first annular portion and the second annular portion. The shells may be arranged along the primary compressive axis such that the first annular portion of a first one of the shells is joined to the first annular portion of a second, adjacent shell of the at least two shells. Such an arrangement of two adjacent shells may define a bellow. The central section extending between the first annular portion and the second annular portion may have a positive outwards curve, i.e. a curve extending outwards of a straight line connecting the first and second annular portions. This positive outwards curve helps to give the shell, and therefore the bellow and so the overall compressive element a desired force response under compression. The compressive element may comprise or consist of polymer material. In some embodiments, the compressive element is arranged such that a compressive force experienced by the compressive element up to a first stress value compresses the compressive element in a first stage of compression by up to a first fraction of an uncompressed length of the compressive element; wherein the compressive element is arranged such that a compressive force above the first stress value and up to a second stress value further compresses the compressive element in a second stage of compression by greater than the first fraction of the uncompressed length of the compressive element and up to a second fraction of the uncompressed length of the compressive element; wherein the compressive element is arranged such that a compressive force above the second stress value further compresses the compressive element in a third stage of compression by greater than the second fraction of the uncompressed length of the compressive element; wherein during the first stage of compression the compressive element exhibits an average stiffness having a first stiffness value, wherein during the second stage of compression the compressive element exhibits an average stiffness having a second stiffness value, and wherein during the third stage of compression the compressive element exhibits an average stiffness having a third stiffness value; and wherein the first stiffness value is greater than the second stiffness value, and the third stiffness value is greater than the second stiffness value. Thus, the compressive element initially compresses very little in the early stages of compression. This allows the compressive element to be compressed to a region with much higher response forces without compressing the element very far and giving up much stroke length. Then, in a second stage of compression, under greater force, the compressive element is able to compress more freely, thus providing a good actuation distance in response to an applied force. Finally, at very high compression forces, the compressive element becomes stiffer again and is very resistant to further compression. This reduces shock in the event of unexpectedly high compression, e.g. in the event of pressure failure. Preferably, the compressive force is of a magnitude to compress the compressive element into the second stage of compression (i.e. disregarding forces due to internal pressure). Thus, the (stress-strain) response of the compressive element is such that there are at least three different stages to the behaviour of the compressive element under compression, i.e. depending on how far it has been compressed compared to its uncompressed length, it produces a different force response to further compression. The use of a compressive element with a known response curve is advantageous since it can be known how much force is required to act on the spring to extend or compress it to a particular length (and therefore provide actuation of a certain magnitude). Therefore, the actuator system can more easily be controlled to accurately produce the desired actuation movement. In some embodiments, the actuator system further comprises a valve, arranged to control the flow of fluid into and / or out of the internal cavity of the compressive element. By this is meant that the valve selectively allows or prevents the flow of fluid into and / or out of the cavity. The use of one or more valves advantageously reduces the amount of time for which the pump needs to run, since a user may simply pump sufficient fluid in or out of the internal cavity to reach the desired extension of the compressive element along the primary compression axis, and then close the valve so as to retain the corresponding amount of fluid within the internal cavity, without the pump continuing to act. The valve may control the flow of fluid both into and out of the internal cavity (i.e. it may be a two-way valve) or alternatively it may control the flow of fluid only into or out of the internal cavity (i.e. it may be a one-way valve). More than one one-way valve may be provided. The compressive element may comprise the valve, i.e. the valve may be located in the side wall of the compressive element (i.e. of the tube defining the compressive element). The valve (or valves) may be the only opening in the internal cavity, i.e. such that internal cavity is sealed other than at the valve(s), so the only route for fluid in or out of the cavity is through the valve(s). In some embodiments, the pump is arranged to pump fluid through the valve. In some embodiments, the actuator system comprises a first end plate, substantially perpendicular to the primary compression axis and positioned to cover a first end of the compressive element, and / or a second end plate, substantially perpendicular to the primary compression axis and positioned to cover a second end of the compressive element (e.g. arranged over opposing ends of the hollow tube). The first end plate and the second end plate may thus be arranged at opposing ends of the internal cavity, sealing the internal cavity (but for any openings or valves in the plates or the compressive element). As set out above, the actuator system may comprise a valve. The first end plate and / or the second end plate may comprise the valve, i.e. such that the valve is located in the end plate. The first end plate and / or the second end plate may be rigid. The first end plate and / or the second end plate may have substantively the same diameter as the diameter of the second annular portions of the two shells, referred to above, e.g. such that the end plates can cover the opening at each end of the compressive element, defined by the second annular portions. The end plates provide a surface to which a mass for actuation may be attached or on which a mass for actuation may be placed. This provides a convenient surface and since it is flat it is relatively insensitive to small variations in angle during actuation. In some embodiments, the actuator system comprises a connecting element (i.e. a connector, also referred to as a connecting means), for connecting the compressive element to an external component, for actuation (i.e. movement) of the external component. The actuator system may further comprise the external component. The connecting element may connect the compressive element directly to the external component (i.e. by directly connecting from one to the other) or the connecting element may connect the compressive element indirectly to the external component, i.e. by connecting between the external component and a component which is itself connected to the compressive element. For example, the connecting element may be connected to the first end plate or the second end plate. The connecting element(s) may be a rigid connecting element or it may be a flexible connecting element(s). The connecting element(s) may be a tension member. The connecting element may provide the compression means (which may also be referred to as a compressor). Thus it may be the connecting element which provides the compressive force, e.g. due to a weight or other force applied by virtue of the external component. In other embodiments, the external component itself may provide the compression means (i.e. it may directly compress the compressive element, without an intervening connecting element), or the compression means may be provided by a frame, as discussed further below. In some embodiments, the actuator system further comprises a fluid connection connected between the pump and the internal cavity (e.g. via the valve). The fluid connection may allow the pump to be located further from the rest of the actuator system (e.g. from the compressive element) than would be possible without the fluid connection. For example, the pump may be mounted to the external component. In some embodiments, the actuation may be provided by a single compressive element (although of course more than one such actuator system may be used). Thus, in some embodiments, the actuator system comprises a single compressive element. In some embodiments, the actuator system further comprises a connection member (e.g. a first compressive-element connection member) and a (first) attachment component, the (first) connection member extending from the first end plate to the (first) attachment component. The (first) connection member may pass through the second end plate, and may do so in a sealed manner (i.e. there may be a seal around the attachment component at the point at which it passes through the second end plate). The (first) attachment component may be positioned adjacent a second end of the compressive element. The connection member may be rigid. The connection member may extend (substantially) parallel to the primary compression axis. The connection member may extend internally within the internal cavity, but preferably extends externally of the internal cavity, to avoid the need for additional seals in the compressive element. In some embodiments, the actuator system further comprises a connection member (e.g. a second compressive-element connection member) and a (second) attachment component, the (second) connection member extending from the second end plate to the (second) attachment component. The (second) connection member may pass through the first end plate, and may do so in a sealed manner (i.e. there may be a seal around the attachment component at the point at which it passes through the first end plate). The (second) attachment component may be positioned adjacent a first end of the compressive element. The connection member may be rigid. The connection member may extend (substantially) parallel to the primary compression axis. The connection member may extend internally within the internal cavity, but preferably extends externally of the internal cavity, to avoid the need for additional seals in the compressive element. The method comprises pumping fluid into the (single) compressive element so as to expand the internal chamber along the primary compression axis. This thereby moves the first end plate and the second end plate apart from each other. This moves the first attachment component and the second attachment component closer together (along the primary compression axis) and therefore moves the external component closer to the fixed anchor. In some embodiments, the actuator system comprises a first connecting element for connecting the compressive element to an external component for actuation of the external component, wherein the first connecting element is connected to an end plate (e.g. the second end plate) of the (single) compressive element. In some embodiments, the actuator system further comprises a second connecting element for connecting the compressive element to an external component for actuation of the external component, wherein the second connecting element is connected to the (second) end plate of the (single) compressive element. This divides the force needed for actuation of the external component between these two connecting elements, and therefore allows each to bear a lower load, reducing the size required of each line. It may also provide redundancy in the case that one of the connecting elements fails. In some embodiments, the actuator system comprises a fixed connecting element, for connecting to an external fixed point (e.g. to a fixed structure or an anchor). The fixed connecting element may be connected to the second attachment component. In some embodiments, the actuator system comprises a secondary connecting element, connecting the compressive element to the external component. The secondary connecting element may be connected to the first attachment component of the compressive element. The method may comprise (partially) compressing the compressive element, and then arranging the compressive element between the fixed connecting element and the secondary connecting element, such that tension applied by the fixed connecting element and the secondary connecting element acts to keep the compressive element in the compressed state (absent internal expansion forces applied for the purpose of actuation). In other embodiments, the actuator system comprises more than one compressive element. Thus, in some embodiments, the compressive element is a first compressive element, and the actuator system further comprises a second compressive element. The first compressive element and the second compressive element may be arranged uniaxially (i.e. with their respective primary compressive axes aligned). Thus, the second compressive element may extend along the primary compression axis and may be configured for compression along the primary compression axis. The second end plate (also referred to as the actuation plate) may be interposed between the first compressive element and the second compressive element (e.g. it may be positioned to cover an end (e.g. the second end) of the first compressive element, and an end (e.g. the first end) of the second compressive element). In some embodiments, the actuator system further comprises a third end plate, substantially perpendicular to the primary compression axis and positioned to cover a second end of the second compressive element (i.e. the opposite end to the end covered by the second end plate). Depending on the order of reference used for the end plates, this may also be referred to as a second end plate (and the intermediate, second end plate instead referred to as an actuation plate). This third end plate may have any of the features discussed above with respect to the first and second end plates. In some embodiments, the actuator system further comprises a frame, connecting the first end plate to the third end plate, such that the first end plate and the third end plate are fixed in position with a fixed separation distance between them along the primary compression axis. Thus, in this example, the frame provides the compression means, holding both of the springs which are arranged between the end plates in compression (by suitable selection of the length of the frame). The frame may be rigid. The frame may comprise one or more elongate members, each of a fixed length and each fixed at a first end to the first end plate and at a second end to the second end plate. The frame (e.g. each elongate member) may pass through an opening in the second end plate (the actuation plate), e.g. with a sliding contact. This may help to stabilise movement of the second end plate and reduce any risk of buckling of the compressive elements. By fixing the first and third end plates relative to one another, the two compressive elements are each held in compression. When one compressive element is expanded by internal pressure it acts against the response force provided by the other compressive element resisting its own compression. In this embodiment, no external compressive force is required, since the frame fixing the end plate separation provides this. Expansion of one compressive element (by internal pressure) is resisted by the reaction force that the other compressive element exhibits as it is compressed due to this expansion. Thus, by knowing and even selecting the response curve of the compressive element(s) that is (or are) to be compressed, the internal pressure required to expand the internal cavity a certain distance, and therefore achieve a desired actuation distance, can be accurately determined. Furthermore, this actuation distance can be achieved effectively independently of the weight to be actuated, provided that the weight is negligible compared to the force responses selected for the compressive elements. By expansion of even one of the first or second compressive elements against the compressive force applied by the other (by virtue of its response force), actuation of a part appropriately connected (directly or indirectly) to that compressive element may be achieved, in line with the description set out above. The second end plate may be an actuation plate. It may be connected to an external component (e.g. by a connecting element) for actuation of that external component. Thus, by appropriate decompression of the first and / or second compressive element, against the response force of the other compressive element, the second end plate may be moved back and forth for actuation of an external component. In some embodiments, the method comprises pumping fluid into the first compressive element so as to expand the first internal cavity along the primary compression axis, thereby moving the actuation plate away from the first end plate. This moves the external component, which is connected to the actuation plate, away from the fixed anchor. The method may further comprise opening a valve of the second compressive element (substantially at the same time as pumping fluid into the first compressive element), so that the second compressive element is able to compress, making it easier for the first internal cavity to expand (by increasing the differential force acting on the actuation plate). Alternatively, there may be no valve on the second compressive element and it may simply be unsealed. Similarly, in some embodiments, the method comprises pumping fluid into the second compressive element so as to expand the second internal cavity along the primary compression axis, thereby moving the actuation plate away from the third end plate. The method may further comprise opening a valve of the first compressive element (substantially at the same time as pumping fluid into the second compressive element), so that the first compressive element is able to compress, making it easier for the second internal cavity to expand. This moves the external component, which is connected to the actuation plate, closer to the fixed anchor. Alternatively, there may be no valve on the first compressive element and it may simply be unsealed. The second compressive element may have any of the features discussed above in relation to the (first) compressive element. Thus, in some embodiments, the second compressive element defines an internal cavity (of the second compressive element) extending along the primary compression axis. The internal cavity of the second compressive element may be referred to as the second internal cavity. In some embodiments, the actuator system comprises a first connecting element for connecting the compressive element to an external component for actuation of the external component, wherein the first connecting element is connected (e.g. attached, directly or indirectly) to the second end plate. In some embodiments, the actuator system further comprises a second connecting element for connecting the compressive element to the external component for actuation of the external component, wherein the second connecting element is connected to the second end plate. The first connecting element and / or the second connecting element may be rigid. In some embodiments, the actuator system comprises a fixed connecting element, for connecting to an external fixed point (e.g. to a fixed structure or an anchor). The fixed connecting element may be connected (e.g. attached, directly or indirectly) to the first end plate. This provides a fixed point of connection to which the compressive element is (effectively) connected, and relative to which the external component is then actuated. The (first and / or second) connecting elements and the secondary connecting element may each be connected to separate mooring points or locations of the external component. This helps to avoid contact between these separate connecting elements. In some embodiments, the actuator system comprises a pump, arranged to pump fluid into the internal cavity of the second compressive element (i.e. the second internal cavity) to increase the length of the second compressive element along the primary compression axis. This pump may be the same pump as the pump which is arranged to pump fluid into or out of the internal cavity of the first compressive element, or it may be a separate pump. In some embodiments, the actuator system may be arranged to compress (i.e. pump fluid out of) one of the first and second compressive elements whilst (substantially) simultaneously decompressing the other of the first and second compressive elements, so as to cause movement of the second end plate between them. This may be particularly effective since the drop in internal pressure in the compressed compressive element further contributes additional force to move the second end plate (the actuation plate) in the desired direction, thus achieving the desired actuation faster. This also prevents the two compressive elements from acting against each other - for example if both compressive elements were expanded at the same time then the pressure in both would increase, but the second end plate between them would not move, and therefore no actuation based on movement of that second end plate would occur. In some embodiments, the first compressive element and the second compressive element may operate as a closed loop. Thus, in some embodiments, the actuator system comprises a valve arranged to control (e.g. to selectively enable or prevent) the flow of fluid between the internal cavity of the first compressive element and the internal cavity of the second compressive element. The internal cavities may otherwise be sealed. Thus, for actuation, the pump(s) and the valve are used to cause fluid to flow from internal cavity of the first compressive element into the internal cavity of the second compressive element, or vice versa. This causes one of the compressive elements (the one into which fluid flows) to expand, whilst the other contracts, actuating the plate located between them which is thereby able to actuate an external component. Such a system is particularly advantageous since any suitable fluid may be used as the fluid within the cavities, without depending on an external supply of said fluid. Avoiding the use of an external fluid supply also reduces the risk of contamination or damage to the actuator system as a result of contaminants in the external fluid supply. In other embodiments, the first compressive element and the second compressive element may operate as an open loop system. Thus, in some embodiments, the actuator system comprises a first valve arranged to control (e.g. to selectively enable or prevent) the flow of fluid into and / or out of the internal cavity of the first compressive element and a second valve arranged to control (e.g. to selectively enable or prevent) the flow of fluid into and / or out of and the internal cavity of the second compressive element. Each valve may connect the respective internal cavity to an external fluid supply, e.g. to a medium in which the compressive element is at least partially immersed. Such a system is advantageous since it should never need to be separately topped up with the fluid, and since it need not require a dedicated fluid supply where the compressive elements are in any case immersed in a suitable fluid (e.g. air or water). In some embodiments, the first compressive element and the second compressive element are substantially identical. By this it will be understood that their shape and material properties are sufficiently close to one another that they each provide approximately the same response to compression (i.e. substantially the same force is required to compress each to the same compression distance). This may advantageously help the actuator system to be capable of providing substantially the same actuation along the primary compression axis in two directions, using substantially the same amount of force (i.e. pressure). Alternatively, the first and second compressive elements may each have different (material) properties. This may allow the actuator system to be biased for actuating in a particular direction (i.e. one way along the primary compression axis) more easily, i.e. requiring a lower force. For example, the first compressive element and the second compressive element may be of different sizes, different shapes, different thicknesses, different materials, and / or may each comprise a different number of bellows (or shells). In some embodiments, the actuator system may comprise more than two compressive elements, i.e. an array of compressive elements. The compressive elements may be arranged in series (i.e. aligned along their respective primary compressive axes). Multiple actuator systems may also be arranged in parallel to provide an actuator system array, e.g. a single end plate may comprise the first and / or second and / or third end plate for a plurality of actuator systems. This allows more precise control of actuation of the appropriate end / intermediate plate. In some embodiments, the fluid is a non-compressible fluid. For example, the fluid may be water (e.g. sea water), or it may be air. The fluid may be provided by a medium in which part or all of the actuator system is immersed (e.g. in which the compressive element and / or the pump is immersed). In some embodiments, the method comprises at least partially (optionally fully) immersing the compressive element(s) into water (e.g. sea water) for operation. Thus, the method comprises pumping water (i.e. from the supply of water in which the compressive element(s) is immersed) into and / or out of the internal cavity. The actuator system may therefore be used in water, e.g. to actuate a system located on or in the water. The use of water or air as the fluid is particularly advantageous since they are readily available in many of the ambient environments in which the actuation system may be used, and since they pose no environmental contamination risk. Features of any aspect or embodiment described herein may, wherever appropriate, be applied to any other aspect or embodiment described herein. Features referenced in relation to the actuator system may likewise be present in the actuator system used in the described method, and similarly where steps are mentioned in the context of the method, the actuator system may be configured or arranged to carry out said steps. Where reference is made to different embodiments or sets of embodiments, it should be understood that these are not necessarily distinct but may overlap. BRIEF DESCRIPTION OF THE DRAWINGS Certain preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an actuator system according to a first embodiment of the present invention, in a compressed configuration; Figure 2 is a schematic diagram of the actuator system of Figure 1 in an actuated position in which the mass has been moved upwards; Figure 3 is a schematic diagram of an actuator system according to a second embodiment of the present invention; Figure 4 is a schematic diagram of the actuator system of Figure 3 in a neutral position; Figure 5 is a schematic diagram of the actuator system of Figure 4 in an actuated position, in which an external component has been moved closer to the fixed reference point by the actuator system; Figure 6 is a schematic diagram of the actuator system of Figure 4 in an actuated position, in which an external component has been moved further away from the fixed reference point by the actuator system; Figure 7 is a schematic diagram of an actuator system according to a third embodiment of the present invention; Figures 8 and 9 are schematic diagrams showing an exemplary use of the actuator system of Figures 1 and 2, in a suspended system; Figures 10 and 11 are schematic diagrams showing another exemplary use of the actuator system of Figures 1 and 2, in this example in a mooring system; Figure 12 is a cross-sectional view of an exemplary compressive element which may be used in the actuator system of Figures 1, 3 or 7; Figure 13 is a graph illustrating the response curve provided by the compressive element of Figure 12 under various stages of its compression; Figure 14 is a graph illustrating the relationship between actuation distance and internal pressure for the actuator system of Figure 3; Figure 15 is a graph illustrating the relationship between actuation distance and actuation force for the actuator system of Figure 3; and Figure 16 is a flow diagram representing a method according to the present invention. DETAILED DESCRIPTION The basic concept underlying the invention may be understood with reference to a particularly simple embodiment shown in Figures 1 and 2. The actuator system 1 of Figures 1 and 2 includes a compressive element 2 and a pump 18. Compressive elements may also be referred to as springs. The compressive element 2 extends along a primary compression axis 14, which in this particular example extends vertically, and it is configured for compression along this primary compression axis 14. The compressive element 2 defines an internal cavity 4 within it, which extends along the primary compression axis 14. The pump 18 is arranged to pump fluid into the internal cavity 4 via a valve 16 to increase the length of the compressive element 2 along the primary compression axis 14. The compressive element 2 has a natural length which it extends when any external forces acting on it are balanced. In order to create a change in length of the compressive element 2 unbalanced forces must act on it. There are two possible options, using the pump 18. Either fluid may be pumped out of the internal cavity 4, to create a lower pressure inside the internal cavity 4 than outside, compressing the compressive element, or fluid may be pumped in to expand the internal cavity 4 and thereby increase the length of the compressive element 2. Reducing the pressure inside the internal cavity (i.e. using negative pressure) is limited by whatever the pressure outside the compressive element 2 is, whereas positive pressure is limited only by the pressure the compressive element 2 is able to withstand. The Applicant has thereby appreciated that driving actuation using positive pressure within the compressive element 2 is preferable. In order to do this, the compressive element 2 is placed under a compressive force 9, so that positive pressure within the compressive element 2 may act against this compressive force and expand the compressive element 2. The compressive force may be considered as being provided by some compression means (i.e. whatever applies the force to compress the compressive element). In this case, as explained below, a mass 7 provides the compression means. In the example of Figures 1 and 2, the internal cavity 4 is sealed at its upper end by a first end plate 6a, which is planar. The compressive element 2 is sealed at a lower end against the ground, or a ground plate 6b, also referred to as a second end plate. Thus, the internal cavity 4 of the compressive element 2 is bounded by the side walls of the first compressive element 2, together with the first end plate 6 and the second end plate 6b. The first end plate 6a is located at a first end 10 of the compressive element 2, and the second end plate 6b is located at a second end 12 of the compressive element 2. Each end plate 6a, 6b is arranged to be substantially perpendicular to the primary compression axis 14. The internal cavity 4 is therefore sealed everywhere apart from at the valve 16. The valve 16 is arranged to selectively allow flow of fluid into or out of the internal cavity 4 with which it is associated. A mass 7 is placed on top of the first end plate 6a. A downwards force 9 (the compressive force) acts on the first end plate 6a as a result of the gravitational force acting downwards on the mass 7. This downwards force (relative to the primary compression axis 14) acts to compress the compressive element 2 along the primary compression axis 14 to the stage of compression shown in Figure 1. As the compressive element 2 is compressed it provides a response force, which increases with the distance of compression. Thus, the point of compression shown in Figure 1 is the compression distance at which the reaction force 11 produced by the spring in response to compression balances with the downwards force 9 created by the weight of the mass 7. To actuate the mass 7 upwards, fluid is pumped into the internal cavity 4 by the pump 18 through the valve 16. This increases the pressure within the internal cavity 4. The internal pressure within the cavity 4 acts outwards on all surfaces of the cavity 4, producing an outwards acting force equal to the product of the internal pressure and the internal area of the cavity 4. The fluid may be supplied from an external reservoir or container (not shown), and returned to that reservoir when pumped out of the compressive element (giving a closed-loop system), or it may be drawn from the medium 132 in which the actuator system 1 is located (e.g. air may be used), and returned to the external environment when pumped out of the compressive element 2. Using the medium in which the actuator system 1 is located as the fluid for providing the internal pressure within the compressive element is convenient and is also environmentally friendly since it poses no contamination risk to the environment surrounding the actuator system 1. The component of this force acting non-vertically (with reference to the view of Figure 2) is resisted by the hoop stress of the compressive element 2, which gives an extremely stiff response. Thus, even a large internal pressure within the cavity 4 changes its shape very little in a direction substantially perpendicular to the primary compression axis 14. By contrast, the compressive element 2 is relatively less stiff along the primary compression axis 14, along which it is configured to compress (and along which it has already been compressed by the mass 7). As described above, the internal pressure within the cavity 4 produces an internal pressure force acting upwards on the first end plate 6a equal to the contact area of the first end plate 6a with the internal cavity 4, multiplied by the internal pressure (F=PxA). This internal pressure force combines with the response force of the compressive element 2 to produce a resultant upwards force 13 acting upwards on the first end plate 6a. This resultant upwards force 13 is greater than the weight 9 of the mass 7, and therefore the first end plate 6a, and with it the mass 7, moves upwards. Thus, the compressive element 2 is able to achieve actuation (i.e. movement) of the mass 7 upwards. If downwards motion is desired, the valve 16 can be opened and fluid can be allowed to leave the internal cavity 4. This will remove the internal pressure force, and therefore the weight of the mass 7 will gradually compress the compressive element 2 back to the position seen in Figure 1. Alternatively, the pump 18 may be used to actively pump fluid out of the internal cavity 4 to achieve this downwards actuation. In the position shown in Figure 1, the reaction force 11 produced by the spring in response to compression is such as to balance the downwards force 9 created by the weight of the mass 7. The forces are thus balanced and so the system is in equilibrium. At this point, only a small increase or decrease in pressure inside the internal cavity 4 is needed to move the first end plate up or down. Thus, a small additional force (created by pressure) may be used to actuate against a large external force acting onto the first end plate 6a. This provides a significant advantage since it provides finer control of the actuation of a large mass with only a small active force. The actuator system 1 illustrated in Figures 1 and 2 has a number of advantages over known actuator systems. Firstly, it is able to provide relatively large actuation range or distance (also referred to as a stroke length), since it can be compressed significantly by the initial force, and then expand all the way out to its original uncompressed length. Since the initial compression under the weight of the mass depends on the response curve of the chosen compressive element, this also gives controllability since the properties of the compressive element can be selected to give it a desired initial compression distance in response to a particular applied weight. Thus, the system is customisable and can be made suitable for any given application and any given mass to be actuated. The response curve of the compressive element can be well known and understood, therefore giving good control of the actuation distance. The system offers further advantages in that it requires relatively few seals, and only one valve. This arrangement is cheaper than known alternatives which need a larger number of components and it is also less prone to failure. Furthermore, the compressive element has a very long fatigue life, since it is designed (e.g. through its shape and choice of material) to withstand many compressions and extensions. Whilst the simple embodiment of Figures 1 and 2 has advantages, as set out above, it will be appreciated that it is also limited in that it relies on the application of an outside force (in this case the weight 9 of the mass 7), in order to actuate the compressive element 2 against this force. The Applicant has appreciated that it is further advantageous to provide an actuator which is capable of actuating a mass without requiring the application of an external force. The concept of such a system may be appreciated with reference to Figure 3. Figure 3 shows an actuator system 101 which includes two compressive elements 102a, 102b. The compressive elements 102a, 102b are arranged vertically one above the other, along the primary compression axis 114. An “end plate”, also referred to as an actuation plate 108, is interposed between the first compressive element 2a and the second compressive element 2b, sealing a lower end of the internal cavity 4a of the first, upper compressive element 2a and sealing an upper end of the internal cavity 4b of the second, lower compressive element 2b. The outer ends of each internal cavity 4a, 4b are closed by respective outer end plates 106a, 106b. Each of the compressive elements 102a, 102b is compressed using a compressive force before then being placed between their two respective end plates. The outer end plates 106a, 106b are then attached to each other by a frame 105. The frame 105 attaches the first outer end plate 6a to the second outer end plate 6b, and holds them at a fixed distance from each other along the primary compression axis 114, thus keeping the compressive elements 102a, 102b compressed. In this example the frame 105 therefore provides the compression means. As explained above, when compressed, each compressive element 102a, 102b, produces a reaction force along the primary compression axis 114 in response to this compression, with the magnitude of the force depending on the compression distance. Since the outer end plates 106a, 106b are fixed in position by virtue of the frame 105 attached between them, the reaction force of each compressive element 106a, 106b act on the central actuation plate 108. In the example of Figure 3 both of the compressive elements 102a, 102b are substantially identical. Therefore, the reaction force produced by each when they are compressed by substantially the same distance is substantially equal, and therefore the forces acting on the central actuation plate 108 are balanced in the position shown in Figure 3. As explained further below, by expanding one of the compressive elements 102a, 102b by pumping in fluid using the respective pump 118a, 118b, the actuation plate 108 may be moved either up or down along the primary compression axis 114, as illustrated by the arrow 107 shown in Figure 3. An embodiment in which two or more springs act against each other is advantageous because it does not require any external force to be applied in order for actuation to be possible, and it is able to actuate the actuation plate 108 a particular distance independently of the weight being actuated by that actuation plate 108. A downwards force 109 acts on the actuation plate 108. This downwards force 109 is the resultant sum of the downwards reaction force produced by the first, upper compressive element 102a due to its compression, and also any internal pressure force due to fluid within the internal cavity 104a of the first compressive element 102a. Similarly, an upwards force 111 acts upwards on the actuation plate, where this force is the sum of the upwards reaction force produced by the second, lower compressive element 102b due to its compression and any internal pressure force due to fluid within the internal cavity 104b of the second compressive element 102b. The resultant forces may therefore be varied by varying the internal pressure within the compressive elements 102a, 102b. The process of actuation using the actuator system 101 is described further with reference to Figures 4-6, which show the actuator system 101 of Figure 3, connected to an external component 122 by a first connecting element 124 and a second connecting element 126, both of which are rigid connecting elements. In particular, both of the connecting elements 124, 126 are connected to the central actuation plate 108. The frame 105 is not shown in Figures 4-6, but it will be understood that some suitable mechanism is provided to fix the distance between the first end plate 106a and the second end plate 106b, for example this may be a cage or frame or other suitable connecting means. Figure 4 shows the actuator system 101 in the neutral position that is shown in Figure 3, in which the pressure in both compressive elements 102a, 102b is equal and therefore the actuation plate 108 is positioned equidistantly between the first outer end plate 106a and the second outer end plate 106b. A fixed connecting element 120 connects the first end plate 106a to a fixed point, such that overall the actuator system 101 is arranged to actuate movement of the external component 122 relative to the fixed point to which the fixed connecting element 120 is attached. Figure 5 illustrates a configuration in which the actuator system 101 has been used to actuate movement of the external component 122 (e.g. mass) to the left, from the perspective of the view of Figure 5. In order to achieve this, the valve of the first (left-hand) compressive element is opened, so that there is no internal pressure due to fluid contained within the first internal cavity 4a. As a result, the force acting to the right on the actuation plate 108 is provided only by the spring response of the first compressive element 102a due to its compression. The valve of the second compressive element 102b is closed, and the pump 118b is used to pump fluid into the internal cavity 104b of the second compressive element 102b. As a result, internal pressure within the second cavity 104b is increased, providing a larger resultant force acting to the left on the actuation plate 108. As a result, the actuation plate 108 is pushed to the left by an actuation distance 103. The actuation distance 103a that the actuation plate 108 moves to the neutral position is determined by the compression distance by which the first compressive element 102a is compressed, which in turn is determined as the compression distance at which the response force provided by that first compressive element 102a under compression is sufficient to balance both the reaction force of the second compressive element and also the force due to the internal pressure within the second compressive element 102b. Figure 6 shows a configuration in which the external component 122 has instead been actuated to move to the right, with reference to the view of Figures 4-6. This process is the reverse of that described above, such that fluid is pumped into the first compressive element 102a, increasing the internal pressure and therefore the force acting to the right on the actuation plate 108. The valve of the second compressive element 102b is opened so that the force acting to the left on the actuation plate 108 is provided only by the spring response of the second compressive element 102b. As a result, the second compressive element 102b is compressed, moving the actuation plate 108 to the right by an actuation distance 103b from the neutral position. As explained above, the actuation distance 103a, 103b is therefore determined by the compression distance that the compressed spring must be compressed to in order to produce a response force which balances the force produced by the internal pressure created in the other spring (plus the force response of that spring). Provided that any mass to be actuated provides a relatively low force to the system, compared to the forces of the two springs acting against each other, then it will be appreciated that a desired actuation distance may be achieved accurately, independently of the size of the mass to be actuated. Large, high force springs may be chosen, and since they act against each other, still only a relatively small change in internal pressure may be needed to compress one spring and achieve actuation. However, the use of high force springs helps to provide weight-independent actuation. This differs from known systems in which the pressure required to actuate a mass a given distance varies with the weight of that mass, such that the weight of the mass must be known in order to accurately move it a desired actuation distance. It be appreciated that although Figure 3 shows the arrangement with the compressive elements vertically on top of each other, this is effectively the same as the configurations shown in Figures 4-6, since the external force of gravity has essentially no effect on the forces within the actuator system. It will furthermore be appreciated that although the compressive elements are illustrated as being substantially the same in this embodiment, they may each have different properties (e.g. different sizes, shapes, response forces under compression etc.), as desired. Furthermore, although actuation in this example is possible in both directions, as illustrated in Figures 5 and 6, in other examples only one compressive element may be provided with a valve and a pump, and the other may not be sealed and may only act by providing a force response under compression. In such a system actuation would only be possible to one side from the neutral position (i.e. the actuation plate 108 could be actuated past the neutral position in a first direction and returned to the neutral position in the second direction, but move no further past the neutral position in the second direction). It will furthermore be understood that although Figure 3 demonstrates an arrangement of two springs to achieve actuation back and forth along a single axis, multiple such pairs may be provided, arranged along perpendicular axes, to enable actuation of a mass in two or three dimensions if desired. Figure 7 shows an actuator system 201 according to a third embodiment of the present invention. Many features of this actuator system 201 are the same as those shown in the first embodiment of Figures 3 to 6 and described in the related text above. Such like components are labelled with the same reference numerals as are used in reference to the first embodiment, but with their value increased by 100. Components which are alike with the first embodiment will not be described again in details. The third embodiment, as with the second embodiment, includes two compressive elements 202a, 202b, each having an internal cavity 204a, 204b. Each internal cavity 204a, 204b has a corresponding external end plate 206a, 206b. There is a second, intermediate end plate 208 (i.e. actuation plate) disposed between the first compressive element 202a and the second compressive element 202b. In the second embodiment, described above, fluid is supplied for pumping into the compressive elements from an external supply, in particular from a medium in which the compressive elements are immersed. Such a system, requiring an external fluid supply, may be referred to as an “open-loop” system. By contrast, the actuator system 201 of Figure 7 is a “closed-loop” system. It does not (usually) require any external supply of fluid, other than where a top-up is needed due to a leakage or loss over time. In order to allow relative expansion and contraction of the two compressive elements 204a, 204b, to actuate movement of the second end plate 208 that is positioned between the compressive elements, the actuator system 201 includes a valve 216 and a pump 218. The valve 216 in this example is positioned in the second end plate 208, passing through the second end plate 208 so as to connect (when in an open configuration) the first internal cavity 204a with the second internal cavity 204b. In this embodiment, to actuate movement of the intermediate plate 208, the valve 216 is opened to enable flow of fluid 232 between the two internal cavities. The pump 218 (which may be located at or adjacent the valve 218 or may be located elsewhere), is activated to pump fluid 232 into whichever compressive element 202a, 202b is desired to expand (i.e. the first compressive element 202a if movement of the second end plate 208 to the right is desired, or the second compressive element 202b if movement of the second end plate 208 to the left is desired). In response the pump 218 pumps fluid 232 through the open valve 216 from one compressive element into the other, creating a pressure differential between the two which acts on the intermediate plate 208 to move it in the same manner as described above. This movement is able to actuate an external component as described above in relation to the second embodiment. Figures 8 and 9 illustrate the actuator system 1 of Figures 1 and 2 incorporated in a suspended system in which a mass 300 is suspended by two suspension members 302, 304. These suspension members 302, 304 are flexible, although they may also be rigid. It is not important in this suspended configuration since in any case they are always kept under tension by the weight of the suspended mass. The mass 300 is suspended from the actuator system 1, and the actuator system 1 (in particular the second attachment component 5b discussed below) is suspended from a fixed suspension point by a fixed suspending member 306. In this example, an external compressing force is again applied which compresses the compressive element 2, and which the compressive element 2 acts against when it expands due to internal pressure. In this example, the external compressing force is applied by the weight of the suspended mass 300 which creates tension in the suspension members 302, 304. In order for the tension in these suspension members 302, 304 to be transferred to the compressive element 2 as compression, two pairs of connection members 3a, 3b are included. The first pair of connection members 3a each extend from the first end plate 6a to a first attachment component 5a (e.g. a padeye), positioned adjacent a second end 12 of the compressive element 2. The first pair of connection members 3a each pass through a respective opening in the second end plate 6b. The second pair of connection members 3b extends from the second end plate 6b to a second attachment component 5b (e.g. a padeye), positioned adjacent a first end 10 of the compressive element 2. To avoid the need for seals, these connection members are arranged externally of the compressive element 2, although it will be appreciated that for a more compact arrangement they could instead pass internally within the cavity 4, and through the opposing end plate. All of the connection members extend substantially parallel to the primary compression axis 14. When the valve 16 is open, the only force acting on the compressive element is the downwards force on the first end plate 6a, created by the weight of the suspended mass 300, since there is no force on the upper end plate due to an internal pressure within the cavity 4. Due to this downwards force, the compressive element 2 is compressed along the primary compression axis 14. As a result of this “pre-compression” the pairs of internal connection members 3a, 3b extend past the respective end plates which they pass through, as seen in Figure 8. It is possible to raise the height of the mass 300 by using the actuator system. Figure 9 illustrates the actuator system 1 in a position in which the suspended mass 300 has been moved upwards, closer to the fixed suspension element 306. In order to achieve this, fluid has been is pumped into the internal cavity 4 from an external medium using the pump (not shown in this Figure). As explained above, this creates an outward force acting on all inner surface of the internal cavity 4. Since the stiffness of the compressive element 2 in the direction perpendicular to the primary compression axis 14 is much higher than along the primary compression axis 4, this pressure creates little or no change in the size of the compressive element 2 perpendicular to the primary compression axis 14. It does however change the length of the compressive element 2 along the primary compressive axis, since the upwards force on the upper end plate 6a becomes large enough to exceed the downwards force on the upper end plate 6a due to the weight of the suspended mass 300. The first pair of connection members 3a are connected to the first end plate 6a, and so as the first end plate 6a moves upwards, so too does the first pair of connection members 3a, and the first attachment component 5a to which it is attached. The suspended mass 300 is attached to this first attachment component 5a, by the suspension members 302, 304 and it is therefore also raised up. Second end plate 6b is likewise pushed downwards by the response force of the compressive element 2 and by the internal pressure, however it is connected via the second pair of connection members 3b to the second attachment component 5b which in turn is connected to the fixed suspension member 306, and therefore it is prevented from moving downwards. This ensures that all of the movement created by the expansion of the compressive element 2 is converted into desired actuation of the suspended mass. Figures 10 and 11 illustrate another use for the actuator system of Figures 1 and 2, in this case incorporated into a mooring system. In this case a fixed anchor mass 401 is provided, to which fixed connecting elements 406a, 406b are connected. Each fixed connecting element 406a, 406b is connected to a second attachment component 5b of a respective actuator system 1. The first attachment component 5a of each actuator system 1 is then connected to a floating mass 400, such that the floating mass 400 is able to be actuated. This system operates in an analogous manner to the system shown in Figures 8 and 9, but with the tension in the suspension members 402, 404 created by the upwards force applied by the floating mass 401 (i.e. by its buoyancy), rather than a downwards force of weight. In the configuration of Figure 10, no internal pressure is present within the internal cavities of each compressive element, so each is compressed by the force in the suspension members 402, 404 and thus the first end plate 6a has moved upwards, allowing the floating mass 400 to move upwards, above the waterline 405. In the configuration of Figure 11, the compressive elements 2 have both been expanded by internal pressure, as explained further above, causing the first end plate 6a of each actuator system to move downwards, thereby moving the floating mass 402 downwards, just below the waterline 405. It will be appreciated that the actuator system can thereby usefully be used to adjust the floating depth of a floating mass 400, whilst achieving the advantages set out above. It will furthermore be appreciated that although two actuator systems are shown here more or fewer may be used depending on the needs of a given system. These may be arranged in a line or along different axes, and they need not always be actuated together. For example, only one of the actuator systems of Figures 10 and 11 may be actuated so as to make the floating mass 402 lie at an angle in the water. The system of Figures 10 and 11 may be located in a marine environment, or in a freshwater environment. Each actuator system may advantageously be an open-loop system (connected to an external fluid supply) and that fluid supply may be the water in which the actuator system is submerged. This avoids the need for sealing fluid within a system, and for topping up fluid within a sealed system, and it also avoids the environmental risk of contamination in the case of a fluid leakage. It will be appreciated that as the water level changes due to weather conditions, e.g. as waves pass the floating mass 400, the upwards force from the floating mass 400 will change, and the compressive elements 2 will change length as a result Although not forming part of the present invention, it will be appreciated that in an alternative arrangement of the described compressive element this change in length may be used in order to operate the compressive elements effectively as pumps, pumping out water when they are compressed. Alternatively, should it be desired that the position of the float 400 is not changed under varying sea conditions, internal pressure may be provided in both of the compressive elements and then each may be sealed, fixing the length of the actuation system (since the fluid is incompressible). The same may likewise be achieved in the two-spring embodiment of Figure 3, by creating the same internal pressure in both internal cavities 104a, 104b (e.g. 1 bar in each), and sealing both, the actuation plate 108 will be unable to move in either direction since to do so would compress the incompressible fluid. This allows the position of the external component to be fixed in place. Figures 12 shows a cross-sectional profile of two adjacent shells 82b, 82b’ of a compressive element 2a, 2b in an uncompressed state which together provide a single bellow 80. It can be seen in Figure 1 that the compressive element 2a includes multiple such bellows (for example two or three). This is also the case for each compressive element 102a, 102b, where multiple are present. In the example of Figure 1, the compressive elements 2a is made of an elastomeric material e.g. polymer. In order to assist in understanding, the dashed line 88 in Figure 12 shows the separation between the upper shell 82b and the lower shell 82b’. This distinction may be merely conceptual, since a series of such shells i.e. a compressive element, may be integrally formed. Each shell 82b, 82b’ comprises a first, outer, annular portion 84, 84’ and a second, inner, annular portion 82, 82’, with a central section 86, 86’ extending between them. The shells 82b, 82b’ are formed by rotating the shell profile, as shown in Figure 12, through 360 degrees around a central axis 90 (the primary compression axis), giving a two-sided symmetric profile shape as shown in Figure 12. One or both of the first, outer, annular portion 84, 84’ and the second, inner, annular portion 82, 82’ may be strengthened. For example, these annular portions 82, 82’, 84, 84’ may be thicker than the central section 86, 86’ of the shell and / or they could be made of a higher grade or stiffer polymer material than the central section 86 of the shell. Figure 13 is a graph illustrating the response curve provided by the compressive element of Figure 1 under various stages of its compression. The response curve can be broken down approximately into three separate stages. The x-axis 46 in Figure 13 shows stress, in arbitrary units, whilst the y-axis 48 represents strain, again in arbitrary units. This stress-strain response may be scaled to a particular value depending on the system in which it is intended to be incorporated. In the first stage 40, up to a first value 45 of the stress, the compressive element 2, 2a, 2b exhibits a high stiffness. This high stiffness causes a small compression of the compressive element to result in a large increase in the force acting outwards on the end plates 6a, 6b, 106a, 106b, 108 attached to the compressive element 2, 2a, 2b. This is advantageous since the compressive element can be moved to a much higher “load regime”, i.e. much higher response force, without giving up too much stroke length of the compressive element. This higher response force allows the actuator to move much larger loads. In the second stage 42, above the first value 45 of the stress and up to a second value 47 of the stress, the compressive element has a gently sloping response curve, thus having a lower stiffness than in the first stage 40. In this second stage 42, a change in resultant pressure acting on the ends of the compressive element 2 will result in a small but appreciable change in the length of the tension element 2 along the primary compression axis. The compressive element is generally operated in this stage during normal actuation operations. It is advantageous since it is an approximately linear response curve and therefore it is easy to determine the required internal pressure to achieve a desired actuation distance, and so user control is easier. Furthermore, since the response curve is a gentle slope there will not be a large change in length in response to a small change in force, allowing for reasonably accurate actuation to be achieved even if the internal pressure change is not very precisely controlled. In the third stage 44 of the stress-strain response curve, above the second value 47 of the stress, the compression of the compressive element is large (i.e. it has been compressed a long way). In the third stage 44, the compressive element 2 exhibits a high stiffness once again, such that a small compression of the compressive element 2 results in a large increase in force acting outwards on the end plates. This higher stiffness may be achieved by adjacent shells 82, 82b’ (specifically their respective central sections 86, 86’) coming into contact under a high degree of compression. This part of the response curve defines the compression distance at which the component resists further compression. For the two-spring arrangement this therefore defines the maximum stroke or actuation distance in one direction (when the compressing spring for that direction is fully compressed). This helps to stop compression of the spring in a controlled manner such as to minimize shock, and can be particularly important where the spring is under an unexpected load, e.g. if the internal pressure fails, or if an unexpectedly high compression force is applied to the compressive element. Figure 14 is a graph illustrating the relationship between actuation distance and internal pressure for the actuator system of Figure 3. The x-axis 50 shows actuator displacement (i.e. displacement of the second end plate 108) in units of mm from a neutral, central position, as illustrated in Figures 3 and 4. Negative displacement indicates displacement of the second end plate 108 (the intermediate plate) to the left, as shown in Figure 5, and positive displacement indicates displacement of the second end plate 108 to the right, as shown in Figure 6. The y-axis 52 illustrates actuator pressure in units of bar, where this actuator pressure is the net pressure acting on the second end plate 108 (i.e. the actuation plate), taking account of the pressure acting from within each of the internal cavities 104a, 104b. The relationship between these two parameters is represented by a solid line 54. It can be seen that at higher displacements, in either direction, the net pressure needed to be acting on the intermediate plate 108 increases more sharply than at low displacements. It can also be seen that the response is symmetrical for displacement in either direction along the primary compression axis. This is the case since in this particular actuator system 101 the compressive elements 102a, 102b are substantially identical, and therefore the same force is needed for the same displacement in either direction from the centre. Figure 15 similarly shows the relationship between actuator displacement in units of mm, along the x-axis 60 with net force acting on the second end plate 8 in units of kiloNewtons (kN) shown along the y-axis 62. The relationship between these two parameters is represented by a solid line 64. Figure 16 is a flow diagram illustrating a method according to an embodiment of the present invention. In a first (optional) stage 160, the one or more compressive elements are at least partially immersed into a fluid (e.g. water). At stage 161, the one or more compressive elements are compressed by a compressive force. They may either be compressed by an applied weight, directly (e.g. as in Figures 1 and 2) or indirectly (as in Figures 8 and 9), some other directional force (e.g. due to buoyancy of a weight as shown in Figures 10 and 11) or by manual compression and subsequent fixing within a structure of fixed length (e.g. as illustrated in Figure 3). It will be appreciated that although shown as following stage 160, this compression step may actually take place before stage 160, or simultaneously with it. Then, at stage 162, fluid is pumped into an internal cavity of one of the compressive elements to increase the length of that compressive element along the primary compression axis, acting against the applied compressive force. The pumped fluid may be from the external fluid in which the compressive elements are (at least partially) immersed, from a separate external supply, or may be internal fluid within the compressive element (e.g. sealed within a closed loop system). The method may further include any of the features and sets discussed above. It will be appreciated by those skilled in the art that the invention has been illustrated by describing one or more specific embodiments thereof, but is not limited to these embodiments; many variations and modifications are possible, within the scope of the accompanying claims.

Claims

1. An actuator system, comprising:a compressive element, extending along a primary compression axis and configured for compression along the primary compression axis, the compressive element defining an internal cavity extending along the primary compression axis;a compression means, arranged to apply a compressive force to the compressive element, along the primary compression axis; anda pump, arranged to pump fluid into the internal cavity to increase the length of the compressive element along the primary compression axis.

2. The actuator system of claim 1, wherein the compressive element extends at least as far along the primary compression axis as the compressive element extends perpendicular to the compression axis.

3. The actuator system of claim 1 or 2, wherein the actuator system is arranged to provide a maximum actuation distance along the primary compression axis which is larger than a width of the compressive element perpendicular to the primary compression axis.

4. The actuator system of any preceding claim, wherein the compressive element is a hollow tube having a shaped or curved profile along the primary compression axis.

5. The actuator system of claim 4, wherein the compressive element comprises at least one bellow.

6. The actuator system of any preceding claim, wherein the compressive element comprises at least two shells, each shell comprising:a first annular portion;a second annular portion; anda central section;wherein the first annular portion and the second annular portion each lie in a plane that is substantially perpendicular to the primary compression axis, wherein the first annular portion has a maximum dimension in a direction substantially perpendicular to the primary compression axis that is greater than a maximum dimension of the second annular portion in a direction substantially perpendicular tothe primary compression axis, and wherein the central section connects and extends between the first annular portion and the second annular portion, wherein the shells are arranged along the primary compressive axis such that the first annular portion of a first one of the shells is joined to the first annular portion of a second, adjacent shell of the at least two shells.

7. The actuator system of any preceding claim, wherein the compressive element comprises polymer material.

8. The actuator system of any preceding claim, wherein the compressive element is arranged such that a compressive force experienced by the compressive element up to a first stress value compresses the compressive element in a first stage of compression by up to a first fraction of an uncompressed length of the compressive element;wherein the compressive element is arranged such that a compressive force above the first stress value and up to a second stress value further compresses the compressive element in a second stage of compression by greater than the first fraction of the uncompressed length of the compressive element and up to a second fraction of the uncompressed length of the compressive element;wherein the compressive element is arranged such that a compressive force above the second stress value further compresses the compressive element in a third stage of compression by greater than the second fraction of the uncompressed length of the compressive element;wherein during the first stage of compression the compressive element exhibits an average stiffness having a first stiffness value, wherein during the second stage of compression the compressive element exhibits an average stiffness having a second stiffness value, and wherein during the third stage of compression the compressive element exhibits an average stiffness having a third stiffness value; andwherein the first stiffness value is greater than the second stiffness value, and the third stiffness value is greater than the second stiffness value.

9. The actuator system of claim 8, wherein the compressive force is of a magnitude to compress the compressive element into the second stage of compression.

10. The actuator system of any preceding claim, further comprising a valve, arranged to control the flow of fluid into and / or out of the internal cavity of the compressive element.

11. The actuator system of any preceding claim, further comprising a first rigid end plate, substantially perpendicular to the primary compression axis and positioned to cover a first end of the compressive element.

12. The actuator system of any preceding claim, further comprising a connecting element, for connecting the compressive element to an external component, wherein the connecting element provides the compression means.

13. The actuator system of any preceding claim, wherein the compressive element is a first compressive element, and the actuator system further comprises a second compressive element, the first compressive element and the second compressive element arranged uniaxially.

14. The actuator system of claim 13, further comprising:a first rigid end plate, substantially perpendicular to the primary compression axis and positioned to cover a first end of the first compressive element;a second rigid end plate, substantially perpendicular to the primary compression axis and position to cover a second end of the second compressive element; anda frame connecting the first end plate to the second end plate, such that the first end plate and the second end plate are fixed in position with a fixed separation distance between them along the primary compression axis.

15. The actuator system of claim 14, further comprising an actuation plate, interposed between the first compressive element and the second compressive element, and arranged to cover a second end of the first compressive element and to cover a first end of the second compressive element.

16. The actuator system of claim 15, further comprising a connecting element, for connecting the compressive element to an external component for actuation of the external component, the connecting element connected to the actuation plate.

17. The actuator system of any of claims 13 to 16, wherein the second compressive element defines an internal cavity, extending along the primary compression axis; andwherein the actuator system further comprises a pump, arranged to pump fluid into the internal cavity of the second compressive element to increase the length of the second compressive element along the primary compression axis.

18. The actuator system of any of claims 13 to 17, wherein the first compressive element and the second compressive element have different sizes, different shapes, different thicknesses, and / or are made of different materials.

19. The actuator system of any of claims 13 to 18, further comprising a valve arranged to control the flow of fluid between the internal cavity of the first compressive element and the internal cavity of the second compressive element, wherein the first compressive element and the second compressive element operate as a closed loop.

20. The actuator system of any of claims 13 to 18, wherein the second compressive element defines an internal cavity, extending along the primary compression axis; andwherein the actuator system further comprises:a first valve, arranged to control the flow of fluid into and / or out of the internal cavity of the first compressive element, the first valve connecting the internal cavity of the first compressive element to an external fluid supply; anda second valve, arranged to control the flow of fluid into and / or out of and the internal cavity of the second compressive element, the second valve connecting the internal cavity of the second compressive element to an external fluid supply.

21. The actuator system of any of claims 1 to 12, wherein the actuator system comprises a single compressive element, and wherein the actuator system further comprises:a first end plate, substantially perpendicular to the primary compression axis and positioned to cover a first end of the compressive element;a second end plate, substantially perpendicular to the primary compression axis and positioned to cover a second end of the compressive element;a first connection member;a first attachment component;a second connection member; anda second attachment component;the first connection member extending from the first end plate to the first attachment component, passing through the second end plate; andthe second connection member extending from the second end plate to the second attachment component, passing through the first end plate.

22. The actuator system of claim 21, further comprising a first connecting element for connecting the compressive element to an external component for actuation of the external component, wherein the first connecting element is connected to the second end plate of the single compressive element.

23. A method of actuating movement of an external component using an actuator system, the method comprising:compressing a compressive element by applying a compressive force to the compressive element along a primary compression axis along which the compressive element is configured for compression; andpumping fluid into an internal cavity of a compressive element to increase the length of the compressive element along a primary compression axis along which the compressive element is configured for compression.

24. The method of claim 23, comprising at least partially immersing the compressive element into water.

25. The method of claim 24, comprising pumping water from the supply of water in which the compressive element is at least partially immersed into the internal cavity.Application No: GB2406821.5Examiner: Ian ChoiClaims searched: 1-25Date of search: 3 October 2024Patents Act 1977: Search Report under Section 17Documents considered to be relevant:Category Relevant to claims Identity of document and passage or figure of particular relevance X 1-7, 10-23 WO 2005 / 088138 Al (TORINO POLITECNICO) See especially figures and pages 4-6. X 1-7,10-23 US 2023 / 0048924 Al (GAUTIER-LE BOULCH) See especially figures and paragraphs 81-99 X 1-7, 10-23 WO 03 / 033917 Al (TORINO POLITECNICO) See especially figures and pages 6-9. v A 1-7, 10-25 JP 2020060298 A (HITACHI GE NUCLEAR ENERGY LTD) See especially figures 13-16 and paragraphs 16-20, 45-61. X 1-7, 10-23 WO 2016 / 123592 Al (SUNFOLDING INC) See especially figures 19a-22c, 32-34b, and paragraphs 65 and 75. X 1-7, 10-25 US 2008 / 0272351 Al (BHACHU et al.) See especially figures 8a-9b, and paragraphs 26-42. X 1-7, 10-14, 17-25 US 2024 / 0009866 Al (GRIFFITH et al.) See especially figures 4-11, 18-20, 25 and 26, and paragraphs 46, 48, 54, 59, 68, 83, 88 and 90.Categories:v Au Document indicating lack of novelty or inventive step A Document indicating technological background and / or state of the art. Y Document indicating lack of inventive step if p Document published on or after the declared priority date but combined with one or more other documents of same category. before the filing date of this invention. & Member of the same patent family E Patent document published on or after, but with priority date earlier than, the filing date of this application.Field of Search:The following online and other databases have been used in the preparation of this search reportSEARCH-PATENTInternational Classification:Subclass Subgroup Valid From F15B 0015 / 10 01 / 01 / 2006 F16J 0003 / 04 01 / 01 / 2006 F16J 0003 / 06 01 / 01 / 2006 B66F 0003 / 35 01 / 01 / 2006

Images